The present invention relates to a thin film cathode, a process or method for manufacturing the thin film cathode and a display device, and more particularly, to a technique which can be effectively applied to a thin film cathode which has a three-layer structure of a base electrode, an insulator layer and a top electrode and wherein an anodic oxide is used as the insulator layer.
A thin film cathode is arranged, for example, so that a voltage is applied between top and base electrodes of a three-layer thin film structure of the top electrode, insulator layer and base electrode to emit electrons into a vacuum space from a surface of the top electrode.
A metal-insulator-metal (MIM) type and a metal-insulator-semiconductor (MIS) type thin film cathode, wherein a metal, insulator and metal layers are laminated and wherein a metal, insulator and semiconductor layers are laminated, respectively, are known as ones of such thin film cathodes.
The MIM type thin film cathode is described, e.g., in JP-A-7-65710.
FIG. 14 is a diagram for explaining the operational principle of a thin film cathode.
When a driving voltage Vd is applied from a driving voltage source to between a top electrode 13 and a base electrode 11 so that an electric field becomes about 1-10 MV/cm within a tunneling insulator 12, electrons in the vicinity of a Fermi level in the base electrode 11 are transmitted through a barrier by a tunneling effect, injected into a conduction band of the tunneling insulator 12 and top electrode 13 to be formed as hot electrons.
Ones of these hot electrons having energies not smaller than a work function xcfx86 of the top electrode 13 are emitted into a vacuum space 18.
In this case, when a plurality of the top electrodes 13 and a plurality of the base electrodes 11 are provided in the form of lines so that these top and base electrode lines intersect with each other and thus a thin film cathode is provided in the form of a matrix; an electron beam can be generated from an arbitrary location. As a result, the thin film cathode can be used as an electron source of a display device or be applied as an electron source of an electron beam lithography system.
Electron emission has been observed conventionally from a metal-insulator-metal (MIM) type structure of gold (Au), aluminum oxide (referred to as Al2O3, hereinafter) and aluminum (referred to as Al, hereinafter) or the like.
In general, a high quality of tunneling insulator 12 for a thin film cathode is made of an anodic oxide.
In particular, a barrier type (no-porous) anodic oxide of Al or Al alloy is uniform in its thickness and can be formed in a defect-free insulator having a high breakdown voltage and a large surface area.
For this reason, this is most suitable as a method for forming an insulator in a thin film cathode mainly applied to a display device or the like.
However, the anodic oxidation method for forming an anodic oxide has a defect that, since this method is a wet oxidation method in an electrolyte, impurities tend to be easily introduced in the film.
FIG. 15 shows, in a model form, a method for forming an anodic oxide.
Anodic oxidation can be advanced by using the base electrode 11 as an anode in an electrolyte 21 and a mesh electrode 22 of platinum (Pt) or the like as a cathode and by applying an anodizing voltage Vox between them from an anodizing voltage source.
In an interface between the base electrode 11 of Al being anodically oxidized and the tunneling insulator 12 of Al2O3, a reaction of oxygen ions (O2xe2x88x92) supplied from the electrolyte 21 with the Al material of the base electrode causes oxidation to progress.
In an interface between the tunneling insulator 12 of Al2O3 and the electrolyte 21, further, a reaction of aluminum ions (Al3+) supplied from the base electrode (Al electrode) 11 with oxygen ions (O2xe2x88x92) supplied from the electrolyte 21 causes the Al2O3 insulator to grow.
In this way, the growth of the Al2O3 film as the tunneling insulator 12 takes place in the two interfaces. However, the growth in the interface between the base electrode 11 and tunneling insulator 12 occurs in an environment free of impurities other than Al and oxygen (O) and thus a relatively pure Al2O3 film can grow. Meanwhile, in the interface between the tunneling insulator 12 and electrolyte 21, electrolyte""s anions 24 in the electrolyte 21 are incorporated into Al2O3 to grow an Al2O3 film containing lots of impurities.
Accordingly the tunneling insulator 12 has a double structure of a less-impurity inner layer 25 of an insulator provided inside of a surface position at the time of starting the anodic oxidation and a more-impurity outer layer 26 of the insulator provided outside thereof.
A ratio in film thickness between the inner and outer layers is determined by a transport number of aluminum ions (Al3+) and oxygen ions (O2xe2x88x92) during the anodic oxidation and by the type of the anodizing electrolyte.
In the case of an Al2O3 barrier type anodic oxide prepared with use of an electrolyte of organic acid such as ammonium salt of tartaric acid or citric acid and with use of nonaqueous solvent such as ethylene glycol, a transport number of ammonium ions (Al3+) is 0.6 and a transport number of oxygen ions (O2xe2x88x92) is 0.4.
Therefore the film thickness ratio of the outer layer 25 to the insulator reaches 60% and the insulator""s outer layer 25 contains carbon as impurities.
Similarly, when an aqueous solution of ammonium borate capable of forming the barrier type anodic oxide is used as the anodizing electrolyte, main impurities are boron, the transport number of aluminum ions (Al3+) is 0.4 and the transport number of oxygen ions (O2xe2x88x92) is 0.6.
Even in this case, the film thickness ratio of the outer layer 25 to the insulator is 40%.
FIG. 16 shows results when a mixture solution of aqueous tartaric acid ammonium and ethylene glycol is used, and when the composition of a tunneling insulator in a thin film cathode formed by an anodic oxidation method is analyzed and measured by a glow discharge spectroscopy.
The amount of carbon as impurities in the tunneling insulator is much in a region about 60% on its surface side, stepwise decreases in the tunneling insulator, and is less in a region about 40% inside thereof.
In this way, the double structure of the tunneling insulator 12 is clearly shown by analysis in a composition depth direction.
FIG. 17 shows a conduction band when such a thin film cathode is operated.
Electrons injected from the base electrode 11 by a tunnel phenomenon of Fowler-Nordheim run through the conduction band of the tunneling insulator 12 and reach the top electrode 13.
At this time, electrons run through the insulator""s outer layer 26. However, since the insulator""s outer layer 26 contains more impurities, the amount of structural fault becomes much and the amount of electron trap 27 becomes much.
As the number of electrons trapped in the insulator""s outer layer 26 becomes large, an electric field within the tunneling insulator 12 becomes low on its base electrode 11 side and high on its top electrode 13 side as shown in FIG. 17.
In this case, since the tunnel injection electric field is relaxed, a diode current decreases and thus an emission current also decreases.
Further, since an electric field becomes locally strong in the vicinity of an interface between the insulator""s outer layer 26 and top electrode, this leads to destruction of the tunneling insulator 12, thus reducing the reliability of a thin film cathode.
The present invention has been made for the purpose of solving the problems in the prior art. It is therefore an object of the present invention to provide a technique for a thin film cathode which can reduce electron trap in an outer layer having much impurity within an insulator formed by an anodic oxidation method to thereby prevent decrease of an emission current and reduction of reliability.
Another object of the present invention is to provide a technique for manufacturing a thin film cathode, which can reduce a ratio of an outer layer having much impurity within an insulator formed by an anodic oxidation method.
A further object of the present invention is to provide a technique wherein a thin film cathode is used in a display device to thereby provide less brightness reduction and a long life to the display device.
The above and other objects and novel features of the present invention will be apparent from description of the present specification and accompanying drawings.
As mentioned above, the thickness of an anodic oxide and a film thickness ratio between insulator""s outer and inner layers are determined based on the applied anodizing voltage, the material of a base electrode, the transport number of oxygen ion, and the type of an anodizing electrolyte.
The inventors of the present invention have paid attention to this respect, and investigated to seek how to increase the ratio of the insulator""s inner layer having less impurity in an anodic oxide, as a method for reducing a ratio of an insulator""s outer layer having much impurity in the anodic oxide, while holding the feature of the anodic oxide that can be formed as the defect-free insulator having a uniform thickness, a high breakdown voltage and a large surface area.
As a result, the inventors have found an effective method of forming an insulator made of an anodic oxide on a surface of a base electrode by (etchback step) etching to remove a surface side of the anodic oxide formed on the surface of the base electrode by an anodic oxidation method and thereafter by a step of again performing the anodic oxidation at least once.
The inventors have also found it especially effective that the thickness of the anodic oxide to be formed by the anodic oxidation method on the surface of the base electrode is made larger than a final specification thickness of the insulator at the time of the first anodic oxidation and thereafter the surface side of the anodic oxide is etched and removed, whereby the anodic oxide is made smaller than the final specification thickness of the insulator, and an insulator made of an anodic oxide having the final thickness is again formed by the anodic oxidation method.
That is, the present invention is featured by a thin film cathode which includes an electron emitter of a three-layer thin film structure comprising a base electrode, a top electrode and an insulator provided between the base and top electrodes and formed from an anodic oxide of the base electrode, the electron emitter emitting electrons from a surface of the top electrode when a positive-polarity voltage is applied to the top electrode, and wherein the insulator contains impurities a concentration of which decreases stepwise from the top electrode toward the base electrode and, wherein when assuming a boundary of center of the impurities stepwise decreasing region, forming as an outer layer the top electrode side of the insulator and forming as an inner layer the base electrode side of the insulator with respect to the boundary, a film thickness ratio of the insulator""s outer layer to the film thickness of the insulator is smaller than 40%.
Further, the present invention is featured by a thin film cathode which includes an electron emitter of a three-layer thin film structure comprising a base electrode, a top electrode and an insulator provided between the base and top electrodes and formed from an anodic oxide of the base electrode, the electron emitter emitting electrons from a surface of the top electrode when a positive-polarity voltage is applied to the top electrode, and wherein, when a test is conducted wherein an initial current flowing from the top electrode to the base electrode is set to have a current density of 0.2 A/cm2 and a D.C. voltage is applied between the top and base electrodes of the electron emitter, a current flowing from the top electrode to the base electrode is not smaller than 50% of the initial current after 2 hours of the test.
The present invention is featured by a thin film cathode which includes an electron emitter of a three-layer thin film structure comprising a base electrode, a top electrode and an insulator provided between the base and top electrodes and formed from an anodic oxide of the base electrode, the electron emitter emitting electrons from a surface of the top electrode when a positive-polarity voltage is applied to the top electrode, and wherein, when a test is conducted wherein an initial current flowing from the top electrode to the base electrode is set to have a current density of 0.2 A/cm2 and a D.C. voltage is applied between the top and base electrodes of the electron emitter, a shift in a current-driving voltage characteristic is shifted to its high voltage side after two hours with the shift being 0.5 V or less.
Furthermore, the present invention is featured by a thin film cathode which includes an electron emitter of a three-layer thin film structure comprising a base electrode, a top electrode and an insulator provided between the base and top electrodes and formed from an anodic oxide of the base electrode, the electron emitter emitting electrons from a surface of the top electrode when a positive-polarity voltage is applied to the top electrode, and wherein, when a constant current pulse test is conducted wherein a pulse voltage is applied between the top and base electrodes of the electron emitter with a current flowing from the top electrode to the base electrode having a peak current density of 0.2 A/cm2 and with a duty ratio of 1/18.3, an increase in a driving voltage after 714 hours (after 10,000 hours, conversion hours, in the case of a duty ratio of 1/256) is 0.5 V or less.
In addition, the present invention is featured by a method for manufacturing a thin film cathode which includes an electron emitter of a three-layer thin film structure comprising a base electrode, a top electrode and an insulator provided between the base and top electrodes and formed from an anodic oxide of the base electrode, the electron emitter emitting electrons from a surface of the top electrode when a positive-polarity voltage is applied to the top electrode, the method comprising:
a first step of forming an anodic oxide on the surface of the base electrode by an anodic oxidation method at the time of forming an insulator on the surface of the base electrode;
a second step of etching and removing a surface side of the anodic oxide formed in the first step; and
a third step of again forming an anodic oxide on the surface of the base electrode by the anodic oxidation method after the second step, the first to third steps being carried out at least once.
The present invention is also featured by a method for manufacturing a thin film cathode which includes an electron emitter of a three-layer thin film structure comprising a base electrode, a top electrode and an insulator provided between the base and top electrodes and formed from an anodic oxide of the base electrode, the electron emitter emitting electrons from a surface of the top electrode when a positive-polarity voltage is applied to the top electrode, the method comprising:
a first step of forming an anodic oxide thicker than a final specification thickness of the insulator on the surface of the base electrode by an anodic oxidation method;
a second step of etching and removing a surface side of the anodic oxide formed in the first step; and
a third step of forming an anodic oxide of the final specification thickness on the surface of the base electrode by the anodic oxidation method again after the second step to form the insulator.
The present invention is also featured by forming the anodic oxide 2.5 times thicker than the final specification thickness in the first step.
The present invention is further featured by the base electrode which is made of aluminum or aluminum alloy.
The present invention is featured by a display device comprising:
a first substrate having a cathode array;
a frame; and
a second substrate having a phosphor pattern, and wherein a space defined by the first substrate, frame and second substrate is a vacuum atmosphere, and the cathode array of the first substrate is made of any of the above-mentioned thin film cathodes.